Influence of biogenic Fe(II) on the extent of microbial reduction of Fe(III) in clay minerals nontronite, illite, and chlorite

Microbial reduction of Fe(III) in clay minerals is an important process that affects properties of clay-rich materials and iron biogeochemical cycling in natural environments. Microbial reduction often ceases before all Fe(III) in clay minerals is exhausted. The factors causing the cessation are, however, not well understood. The objective of this study was to assess the role of biogenic Fe(II) in microbial reduction of Fe(III) in clay minerals nontronite, illite, and chlorite. Bioreduction experiments were performed in batch systems, where lactate was used as the sole electron donor, Fe(III) in clay minerals as the sole electron acceptor, and Shewanella putrefaciens CN32 as the mediator with and without an electron shuttle (AQDS). Our results showed that bioreduction activity ceased within two weeks with variable extents of bioreduction of structural Fe(III) in clay minerals. When fresh CN32 cells were added to old cultures (6 months), bioreduction resumed, and extents increased. Thus, cessation of Fe(III) bioreduction was not necessarily due to exhaustion of bioavailable Fe(III) in the mineral structure, but changes in cell physiology or solution chemistry, such as Fe(II) production during microbial reduction, may have inhibited the extent of bioreduction. To investigate the effect of Fe(II) inhibition on CN 32 reduction activity, a typical bioreduction process (consisting of lactate, clay, cells, and AQDS in a single tube) was separated into two steps: (1) AQDS was reduced by cells in the absence of clay; (2) Fe(III) in clays was reduced by biogenic AH₂DS in the absence of cells. With this method, the extent of Fe(III) reduction increased by 45-233%, depending on the clay mineral involved. Transmission electron microscopy observation revealed a thick halo surrounding cell surfaces that most likely resulted from Fe(II) sorption/precipitation. Similarly, the inhibitory effect of Fe(II) sorbed onto clay surfaces was assessed by presorbing a certain amount of Fe(II) onto clay surfaces followed by AH₂DS reduction of Fe(III). The reduction extent consistently decreased with an increasing amount of presorbed Fe(II). The relative reduction extent [i.e., the reduction extent normalized to that when the amount of presorbed Fe(II) was zero] was similar for all clay minerals studied and showed a systematic decrease with an increasing clay-presorbed Fe(II) concentration. These results suggest a similar inhibitory effect of clay-sorbed Fe(II) for different clay minerals. An equilibrium thermodynamic model was constructed with independently estimated parameters to evaluate whether the observed cessation of Fe(III) reduction by AH₂DS was due to exhaustion of reaction free energy. Model-calculated reduction extents were, however, over 50% higher than experimentally measured, indicating that other factors, such as blockage of the electron transfer chain and mineralogy, restricted the reduction extent. Another important result of this study was the relative reducibility of Fe(III) in different clays: nontronite > chlorite > illite. This order was qualitatively consistent with the differences in the crystal structure and layer charge of these minerals.     

Bioreduction of Fe-bearing clay minerals and their reactivity toward pertechnetate (Tc-99)

⁹⁹Technetium (⁹⁹Tc) is a fission product of uranium-235 and plutonium-239 and poses a high environmental hazard due to its long half-life (t_1/2 = 2.13 × 10⁵ y), abundance in nuclear wastes, and environmental mobility under oxidizing conditions [i.e., Tc(VII)]. Under reducing conditions, Tc(VII) can be reduced to insoluble Tc(IV). Ferrous iron, either in aqueous form (Fe²⁺) or in mineral form [Fe(II)], has been used to reduce Tc(VII) to Tc(IV). However, the reactivity of Fe(II) from clay minerals, other than nontronite, toward immobilization of Tc(VII) and its role in retention of reduced Tc(IV) has not been investigated. In this study the reactivity of a suite of clay minerals toward Tc(VII) reduction and immobilization was evaluated. The clay minerals chosen for this study included five members in the smectite-illite (S-I) series, (montmorillonite, nontronite, rectorite, mixed layered I-S, and illite), chlorite, and palygorskite. Surface Fe-oxides were removed from these minerals with a modified dithionite-citrate-bicarbonate (DCB) procedure. The total structural Fe content of these clay minerals, after surface Fe-oxide removal, ranged from 0.7% to 30.4% by weight, and the structural Fe(III)/Fe(total) ratio ranged from 45% to 98%. X-ray diffraction (XRD) and Mössbauer spectroscopy results showed that after Fe oxide removal the clay minerals were free of Fe-oxides. Scanning electron microscopy (SEM) revealed that little dissolution occurred during the DCB treatment. Bioreduction experiments were performed in bicarbonate buffer (pH-7) with structural Fe(III) in the clay minerals as the sole electron acceptor, lactate as the sole electron donor, and Shewanella putrefaciens CN32 cells as a mediator. In select tubes, anthraquinone-2,6-disulfate (AQDS) was added as electron shuttle to facilitate electron transfer. In the S-I series, smectite (montmorillonite) was the most reducible (18% and 41% without and with AQDS, respectively) and illite the least (1% for both without and with AQDS). The extent and initial rate of bioreduction were positively correlated with the percent smectite in the S-I series (i.e., layer expandability). Fe(II) in the bioreduced clay minerals subsequently was used to reduce Tc(VII) to Tc(IV) in PIPES buffer. Similar to the trend of bioreduction, in the S-I series, reduced NAu-2 showed the highest reactivity toward Tc(VII), and reduced illite exhibited the least. The initial rate of Tc(VII) reduction, after normalization to clay and Fe(II) concentrations, was positively correlated with the percent smectite in the S-I series. Fe(II) in chlorite and palygorskite was also reactive toward Tc(VII) reduction. These data demonstrate that crystal chemical parameters (layer expandability, Fe and Fe(II) contents, and surface area, etc.) play important roles in controlling the extent and rate of bioreduction and the reactivity toward Tc(VII) reduction. Reduced Tc(IV) resides within clay mineral matrix, and this association could minimize any potential of reoxidation over long term.     

Reduction of TcO4 by sediment-associated biogenic Fe(II)

The potential for reduction of ⁹⁹TcO₄⁻_(aq) to poorly soluble ⁹⁹TcO₂ · nH₂O_(s) by biogenic sediment-associated Fe(II) was investigated with three Fe(III)-oxide containing subsurface materials and the dissimilatory metal-reducing subsurface bacterium Shewanella putrefaciens CN32. Two of the subsurface materials from the U.S. Department of Energy’s Hanford and Oak Ridge sites contained significant amounts of Mn(III,IV) oxides and net bioreduction of Fe(III) to Fe(II) was not observed until essentially all of the hydroxylamine HCl-extractable Mn was reduced. In anoxic, unreduced sediment or where Mn oxide bioreduction was incomplete, exogenous biogenic TcO₂ · nH₂O_(s) was slowly oxidized over a period of weeks. Subsurface materials that were bioreduced to varying degrees and then pasteurized to eliminate biological activity, reduced TcO₄⁻_(aq) at rates that generally increased with increasing concentrations of 0.5 N HCl-extractable Fe(II). Two of the sediments showed a common relationship between extractable Fe(II) concentration (in mM) and the first-order reduction rate (in h⁻¹), whereas the third demonstrated a markedly different trend. A combination of chemical extractions and ⁵⁷Fe Mössbauer spectroscopy were used to characterize the Fe(III) and Fe(II) phases. There was little evidence of the formation of secondary Fe(II) biominerals as a result of bioreduction, suggesting that the reactive forms of Fe(II) were predominantly surface complexes of different forms. The reduction rates of Tc(VII)O₄⁻ were slowest in the sediment that contained plentiful layer silicates (illite, vermiculite, and smectite), suggesting that Fe(II) sorption complexes on these phases were least reactive toward pertechnetate. These results suggest that the in situ microbial reduction of sediment-associated Fe(III), either naturally or via redox manipulation, may be effective at immobilizing TcO₄⁻_(aq) associated with groundwater contaminant plumes.     

Products of abiotic U(VI) reduction by biogenic magnetite and vivianite

Reductive immobilization of uranium by the stimulation of dissimilatory metal-reducing bacteria (DMRB) has been investigated as a remediation strategy for subsurface U(VI) contamination. In those environments, DMRB may utilize a variety of electron acceptors, such as ferric iron which can lead to the formation of reactive biogenic Fe(II) phases. These biogenic phases could potentially mediate abiotic U(VI) reduction. In this work, the DMRB Shewanella putrefaciens strain CN32 was used to synthesize two biogenic Fe(II)-bearing minerals: magnetite (a mixed Fe(II)-Fe(III) oxide) and vivianite (an Fe(II)-phosphate). Analysis of abiotic redox interactions between these biogenic minerals and U(VI) showed that both biogenic minerals reduced U(VI) completely. XAS analysis indicates significant differences in speciation of the reduced uranium after reaction with the two biogenic Fe(II)-bearing minerals. While biogenic magnetite favored the formation of structurally ordered, crystalline UO₂, biogenic vivianite led to the formation of a monomeric U(IV) species lacking U-U associations in the corresponding EXAFS spectrum. To investigate the role of phosphate in the formation of monomeric U(IV) such as sorbed U(IV) species complexed by mineral surfaces, versus a U(IV) mineral, uranium was reduced by biogenic magnetite that was pre-sorbed with phosphate. XAS analysis of this sample also revealed the formation of monomeric U(IV) species suggesting that the presence of phosphate hinders formation of UO₂. This work shows that U(VI) reduction products formed during in situ biostimulation can be influenced by the mineralogical and geochemical composition of the surrounding environment, as well as by the interfacial solute-solid chemistry of the solid-phase reductant.     

Composition,stability, and measurement of reduced uranium phases for groundwater bioremediation at Old Rifle,CO

Reductive biostimulation is currently being explored as a possible remediation strategy for U-contaminated groundwater, and is being investigated at a field site in Rifle, CO, USA. The long-term stability of the resulting U(IV) phases is a key component of the overall performance of the remediation approach and depends upon a variety of factors, including rate and mechanism of reduction, mineral associations in the subsurface, and propensity for oxidation. To address these factors, several approaches were used to evaluate the redox sensitivity of U: (1) measurement of the rate of oxidative dissolution of biogenic uraninite (UO_2(s)) deployed in groundwater at Rifle, (2) characterization of a zone of natural bioreduction exhibiting relevant reduced mineral phases, and (3) laboratory studies of the oxidative capacity of Fe(III) and reductive capacity of Fe(II) with regard to U(IV) and U(VI), respectively.     

Reduction of structural Fe(III) in nontronite by methanogen Methanosarcina barkeri

Clay minerals and methanogens are ubiquitous and co-exist in anoxic environments, yet it is unclear whether methanogens are able to reduce structural Fe(III) in clay minerals. In this study, the ability of methanogen Methanosarcina barkeri to reduce structural Fe(III) in iron-rich smectite (nontronite NAu-2) and the relationship between iron reduction and methanogenesis were investigated. Bioreduction experiments were conducted in growth medium using three types of substrate: H₂/CO₂, methanol, and acetate. Time course methane production and hydrogen consumption were measured by gas chromatography. M. barkeri was able to reduce structural Fe(III) in NAu-2 with H₂/CO₂ and methanol as substrate, but not with acetate. The extent of bioreduction, as measured by the 1,10-phenanthroline method, was 7-13% with H₂/CO₂ as substrate, depending on nontronite concentration (5-10 g/L). The extent was higher when methanol was used as a substrate, reaching 25-33%. Methanogenesis was inhibited by Fe(III) reduction in the H₂/CO₂ culture, but enhanced when methanol was used. High charge smectite and biogenic silica formed as a result of bioreduction. Our results suggest that methanogens may play an important role in biogeochemical cycling of iron in clay minerals and may have important implications for the global methane budget.     

Assessing the redox properties of iron-bearing clay minerals using homogeneous electrocatalysis

Iron-bearing clay minerals are ubiquitous in the environment and have been shown to play important roles in several biogeochemical processes. Previous efforts to characterize the Fe²⁺–Fe³⁺ redox couple in clay minerals using electrochemical techniques have been limited by experimental difficulties due to inadequate reactivity between clay minerals and electrodes. The current work overcomes this limitation by utilizing organic electron transfer mediators that rapidly transfer electrons with both the Fe-bearing clay minerals and electrodes. Here, an Fe-rich source clay mineral (ferruginous smectite, SWa-1) is examined with respect to what fraction of structural Fe participates in oxidation/reduction reactions and the relationship between bulk Fe²⁺/Fe³⁺ ratios to the reduction potential (E_h).     

The geochemistry of iron in Recent tidal-flat sediments of the Wash area,England: a mineralogical,Mössbauer,and magnetic study

Undisturbed core samples of Recent sediments from the Wash tidal flats, East Anglia, England, obtained using a Delft corer, were studied with special reference to the diagenesis and geochemical behaviour of iron. The Mössbauer effect in ⁵⁷Fe was used to monitor the distribution of Fe between different phases as a function of depth, together with the magnetic mineralogy and palaeomagnetic properties.The cores consist of, successively downwards: 0.36 m brown clay; 1.5 m finely laminated silts and fine sands, and 7.14 m homogeneous fine sands. The dominant minerals are quartz, feldspar, calcite and clay minerals, and chemical analysis for Al, Si, Mg, Mn, Ca, Fe, Na, K showed variations closely linked to lithological changes. Illite is the most abundant clay mineral (mean 48%), followed by mixed layer illite-montmorillonite and montmorillonite, kaolinite and chlorite. Chlorite is the major iron-bearing clay mineral and represents 4 to 10% of the <2 μm fraction throughout the core. Sulphide minerals are present throughout the core, including framboidal pyrite.Computer fit analysis of the Mössbauer spectra of best quality showed contributions from Fe²⁺ and Fe³⁺ in clay minerals (essentially chlorite), low-spin Fe²⁺ in pyrite, and magnetically ordered iron in greigite (Fe₃S₄). Systematic variations, as a function of sample depth, indicate a relative increase in the amount of Fe in pyrite at the expense of the clay minerals.Magnetite and titanium-bearing magnetite are the carriers of natural magnetic remanence in these sediments.The direction and intensity of natural remanence in the samples compare well with the known secular variation of the Earth's magnetic field derived from the historic-archaeomagnetic record and this enables the samples to be dated and sedimentation rates to be determined (1.5 mm yr^?1 for the upper 2 m and ~7.7 mm yr^?1 for the lower 7 m).     

A review of the effects of iron redox cycles on smectite properties

The oxidation state of Fe in soils is one of the few properties that can be altered in situ and is known to have a large effect on chemical and physical properties. Understanding this phenomenon enables the possible manipulation or management of the soil in such a way as to maximize the benefit of such changes in properties. The effects of redox cycles, however, are less understood because most studies have focused only on a single reduction event. The purpose of this report is to review the current state of knowledge of the effects of redox cycles on clay and soil behavior, with the hope that this background will encourage further investigations of these processes. Evidence clearly indicates that the means by which reduction of Fe in clay minerals occurs significantly influences the potential reversibility of the process.     

Characterization of accumulated precipitates during subsurface iron removal

The principle of subsurface iron removal for drinking water supply is that aerated water is periodically injected into the aquifer through a tube well. On its way into the aquifer, the injected O₂-rich water oxidizes adsorbed Fe²⁺, creating a subsurface oxidation zone. When groundwater abstraction is resumed, the soluble Fe²⁺ is adsorbed and water with reduced Fe concentrations is abstracted for multiple volumes of the injection water. In this article, Fe accumulation deposits in the aquifer near subsurface treatment wells were identified and characterized to assess the sustainability of subsurface iron removal regarding clogging of the aquifer and the potential co-accumulation of other groundwater constituents, such as As. Chemical extraction of soil samples, with Acid-Oxalate and HNO₃, showed that Fe had accumulated at specific depths near subsurface iron removal wells after 12 years of operation. Whether it was due to preferred flow paths or geochemical mineralogy conditions; subsurface iron removal clearly favoured certain soil layers. The total Fe content increased between 11.5 and 390.8 mmol/kg ds in the affected soil layers, and the accumulated Fe was found to be 56-100% crystalline. These results suggest that precipitated amorphous Fe hydroxides have transformed to Fe hydroxides of higher crystallinity. These crystalline, compact Fe hydroxides have not noticeably clogged the investigated well and/or aquifer between 1996 and 2008. The subsurface iron removal wells even need less frequent rehabilitation, as drawdown increases more slowly than in normal production wells. Other groundwater constituents, such as Mn, As and Sr were found to co-accumulate with Fe. Acid extraction and ESEM-EDX showed that Ca occurred together with Fe and by X-ray Powder Diffraction it was identified as calcite.     

Iron-oxide crystallinity increases during soil redox oscillations

An Inceptisol A-horizon from Hawaii was subjected to a series of reduction-oxidation cycles—14 d cycle length over a 56 d duration—across the “soil-Fe” [Fe(OH)₃.Fe²⁺_(aq), log K^o = 15.74] equilibrium in triplicate redox-stat reactors. Each reducing event simulated the flush of organic C and diminished O₂ that accompanies a rainfall-induced leaching of bioavailable reductants from the forest floor into mineral soil. The soil contained considerable amounts of short-range ordered (SRO) minerals (e.g., nano-goethite and allophane) and organic matter (11% org-C). Room temperature and cryogenic ⁵⁷Fe Mössbauer spectroscopy showed that the iron-bearing minerals were dominated by nano- to micro-scale goethite, and that ferrihydrite was not present. Over the four full cycles, fluctuations in E_h (from 200 to 700 mV) and pFe²⁺ (from 2.5 to 5.5) were inversely correlated with those of pH (5.5 to 4). Here, we focus on the solubility dynamics of the framework elements (Si, Fe, Ti, and Al) that constitute 35% of the oxygen-free soil dry mass. Intra-cycle oscillations in dissolved (<3 kDa) metals peaked during the reduction half-cycles. Similar intra-cycle oscillations were observed in the HCl and acid ammonium oxalate (AAO) extractable pools. The cumulative response of soil solids during multiple redox oscillations included: (1) a decrease in most HCl and AAO extractable metals and (2) a transformation of SRO Fe (as nano-goethite) to micro-crystalline goethite and micro-crystalline hematite. This may be the first direct demonstration that Fe oxide crystallinity increases during redox oscillations—an a priori unexpected result.     

Rapid clay transformations in Delaware salt marshes

Salt marshes are the major areas for net sedimentation in many estuaries such as the Delaware Bay, and their diagenetic chemistry is harsh and extreme with large seasonal excursions in chlorinity (1–50 ppt), pH (4–6), and Eh (−240+120). Such diagenesis is driven by organic matter decomposition using redox cycles of S and Fe materials imported primarily as tidal sea water SO₄ and Fe silicates, respectively.Important and quantitative changes in clay mineralogy occur within a decade at the redox boundary in a high marsh sediment near Lewes, Delaware. The clay mineralogy consists initially of a micaceous illite and chlorite mixture accumulating at the salt marsh surface. It is comprised of relic glacial sediments deposited on the continental slope during their net tidal movement from the sea to land. Once buried, these detrital clays are transformed into a new assemblage containing an illite/smectitic mixed layer mineral of poor crystallinity. Using curve decomposition techniques on complex X-ray traces, it is estimated that this new phase constitutes 45–55% of the clay fraction.The redox boundary where the sharp transition occurs is only about 20 a old as determined by ²¹⁰Pb and ¹³⁷Cs geochronology, and, thus, the clay mineral transformation is rapid. The occurrence of the new, abundant clay mineral is very abrupt (less than 1 cm at 12 cm in depth) and, thus, may itself occur in as little as three years. Once formed, the new mixed layer phase remains stable during the subsequent 40 a of burial from the time of formation at the oxic/anoxic boundary.Slow transformations of unstable primary clay reactants such as illite and chlorite are a common process of soil formation. However such rapid clay reactions have rarely been documented in either subaerial or submerged soil settings. The formation of a smectite mineral product of high chemical reactivity for a significant portion of the clays in a soil is unusual. In fact, the abrupt change in clay mineralogy in the salt marsh occurs precisely at the sharp evolution in salt marsh geochemistry from oxidized to reducing conditions where there is extensive redox cycling of Fe and S phases. A large seasonal oscillation in interstitial pH and Eh probably contributes to the rapid clay transformation. Such clay transformations may have important implications for the retention of other trace elements entering the salt marsh by atmospheric fallout and tidal cycles, or the release of such metal inventories after burial.     

Fe-solid phase transformations under highly basic conditions

Hyperalkaline and saline radioactive waste fluids with elevated temperatures from S-SX high-level waste tank farm at Hanford, WA, USA accidentally leaked into sediments beneath the tanks, initiating a series of geochemical processes and reactions whose significance and extent was unknown. Among the most important processes was the dissolution of soil minerals and precipitation of stable secondary phases. The objective of this investigation was to study the release of Fe into the aqueous phase upon dissolution of Fe-bearing soil minerals, and the subsequent formation of Fe-rich precipitates. Batch reactors were used to conduct experiments at 50 °C using solutions similar in composition to the waste fluids. Results clearly showed that, similarly to Si and Al, Fe was released from the dissolution of soil minerals (most likely phyllosilicates such as biotite, smectite and chlorite). The extent of Fe release increased with base concentration and decreased with Al concentration in the contacting solution. The maximum apparent rate of Fe release (0.566 × 10⁻¹³ mol m⁻² s⁻¹) was measured in the treatment with no Al and a concentration of 4.32 mol L⁻¹ NaOH in the contact solution. Results from electron microscopy indicated that while Si and Al precipitated together to form feldspathoids in the groups of cancrinite and/or sodalite, Fe precipitation followed a different pathway leading to the formation of hematite and goethite. The newly formed Fe oxy-hydroxides may increase the sorption capacity of the sediments, promote surface mediated reactions such as precipitation and heterogeneous redox reactions, and affect the phase distribution of contaminants and radionuclides.     

Limitations of the ferrozine method for quantitative assay of mineral systems for ferrous and total iron

The quantitative assay of clay minerals, soils, and sediments for Fe(II) and total Fe is fundamental to understanding biogeochemical cycles occurring therein. The commonly used ferrozine method was originally designed to assay extracted forms of Fe(II) from non-silicate aqueous systems. It is becoming, however, increasingly the method of choice to report the total reduced state of Fe in soils and sediments. Because Fe in soils and sediments commonly exists in the structural framework of silicates, extraction by HCl, as used in the ferrozine method, fails to dissolve all of the Fe. The phenanthroline (phen) method, on the other hand, was designed to assay silicate minerals for Fe(II) and total Fe and has been proven to be highly reliable. In the present study potential sources of error in the ferrozine method were evaluated by comparing its results to those obtained by the phen method. Both methods were used to analyze clay mineral and soil samples for Fe(II) and total Fe. Results revealed that the conventional ferrozine method under reports total Fe in samples containing Fe in silicates and gives erratic results for Fe(II). The sources of error in the ferrozine method are: (1) HCl fails to dissolve silicates and (2) if the analyte solution contains Fe³⁺, the analysis for Fe²⁺ will be photosensitive, and reported Fe(II) values will likely be greater than the actual amount in solution. Another difficulty with the ferrozine method is that it is tedious and much more labor intensive than the phen method. For these reasons, the phen method is preferred and recommended. Its procedure is simpler, takes less time, and avoids the errors found in the ferrozine method.     

Using soil survey data for series-level environmental phosphorus risk assessment

Choosing soil series scale for assessing phosphorus (P) retention and release characteristics may help relate routinely collected series-specific soil survey data with P retention and aid in designing series-specific P management strategies. Phosphorus retention and release characteristics of pedons collected from two benchmark upland soil series (Berks and Monongahela) and two floodplain (Huntington and Lindside) soil series of West Virginia (USA) were assessed by evaluating P sorption capacity (PSC, Langmuir method) and its major determinants, and effect of different levels of degree of P saturation (DPS) and soil test P (STP, Mehlich-1 P) on the desorbable P (0.01 M CaCl₂-extractable) concentrations. The PSC of the two floodplain soils, Huntington and Lindside, was similar but lower than PSC of upland Berks and Monongahela soils. However, thicker A horizons of Huntington and Lindside soils may compensate for their lower PSC. The B horizons exhibited higher PSC than A horizons. However, slow permeability and thinness of such horizons may discount the higher PSC effect. Relationship of PSC with ammonium oxalate extractable Al (AOX-Al) and Fe (AOX-Fe), dithionite–citrate–bicarbonate extractable Al (DCB-Al) and Fe (DCB-Fe), total C, clay content, and pH [soil:water ratio 1:1 (pH-water) and soil:0.01 M CaCl₂ solution ratio 1:2 (pH-CaCl₂)] showed that in general all except Fe and total C influenced PSC significantly. Aluminum associated with crystalline clay minerals particularly affected PSC, especially of upland soils. Most of the soils did not release considerable P even beyond the conventional critical limit of 25 % DPS for well-drained soils. DPS-desorbable P relationships, though, reflected poor reliability of DPS as an environmental index. At a given DPS and STP, surface horizons released more P than their subsurface counterparts and thus reflected the net sink character of subsurface horizons. Most of the soils did not show considerable release of P even beyond agronomically high STP levels (>23 mg kg^?1). The study provides an economical alternative to time and money-intensive lysimetric studies for assessing subsurface P loss. It reveals the workability of integrating environmental P studies with soil survey data and superiority of integrated assessment of environmental indices of P over the use of any single index.     

Arsenic behaviour in gold-ore mill tailings,Massif Central,France: hydrogeochemical study and investigation of in situ redox signatures

Historical Au-ore exploitation at the Chéni mine in the Massif Central, France, generated 525,000 tonnes of finely ground mill tailings deposited in a heap that has spread with time into three settling basins. The tailings, which are rich in quartz (80%), mica and clay minerals (10% of illite, smectite, kaolinite and chlorite), feldspars (5%) but poor in carbonates (<1%), also contain sulphides (around 5%, mainly pyrite and arsenopyrite). Arsenic content of the tailings is around 6 g kg. This paper describes the geochemistry of drainage waters, with special attention paid to in situ values of the three major redox couples, namely Fe(II)/Fe(III), As(III)/As(V) and S(IV)/S(VI). The water samples range from acidic and oxidized (pH 2.9, E_h +700 mV) to moderate pH and weakly reducing (pH 7.6, E_h 15 mV). The waters are rich in SO₄ and Ca and have variable As (0.05–95 mg L⁻¹) and Fe concentrations (0.07–141 mg L⁻¹). Reduced As(III) species predominate over As(V) species (As(III)/As(V) up to 21), whereas oxidized forms of Fe and S are favoured (Fe(II)/Fe(III) up to 0.5, and S(IV)/S(VI) up to 1).Thermodynamic calculations were performed with the PHREEQC and EQ3NR codes based on a revised As database to evaluate saturation indices (SI) of the waters in relation to the main minerals and define which redox couples control the redox state of the system. The important role of carbonates, though only present in small amounts, explains the acid buffering generated by the oxidation of sulphides for waters in the pH 7–7.5 range. Measured E_h appears to fall between the calculated E_h of the Fe(II)/Fe(III) couple and that of the As(III)/As(V) couple, illustrating redox disequilibrium.     

Control of Cl production in carbonaceous shales by phosphate minerals

When using ³⁶Cl to date very old groundwater in regional aquifer systems, knowledge of the subsurface ³⁶Cl input into the aquifer system is essential. Although ³⁶Cl can be produced through nuclear reactions in the subsurface, in many situations, the input of ³⁶Cl into sedimentary aquifer systems by this avenue of production can be neglected. This is a valid assumption when investigating long-flowpath groundwater systems composed of sandstones, limestones, and shales of typical composition. These rock types are not sufficiently enriched in radioactive elements to produce significant ³⁶Cl in the deep subsurface. Carbonaceous shales, on the other hand, can concentrate the radioactive elements necessary to produce significant ³⁶Cl in the deep subsurface. Chlorine-36 ratios (³⁶Cl/Cl) for a suite of Late Devonian and Pennsylvanian carbonaceous shales were calculated from bulk-rock chemistry as well as measured using accelerator mass spectrometry. The poor agreement between calculated and measured ratios is the result of the assumption of chemical homogeneity used by the calculation algorithm, an assumption that was not satisfied by the carbonaceous shales. In these shales, organic matter, clay minerals, and accessory minerals are heterogeneously distributed and are physically distinct on a micron-order scale. Although organic matter and clay minerals constitute the overwhelming bulk of the shales, it is the phosphate minerals that are most important in enhancing, and suppressing, ³⁶Cl production. Minerals such as apatite and carbonate-apatite (francolite)—by including uranium, rare earth elements (REEs), and halogens—have an important impact on both neutron production and thermal neutron absorption. By incorporating both uranium and fluorine, phosphate minerals act as neutron production centers in the shale, increasing the probability of ³⁶Cl production. By incorporating REEs and chlorine, phosphate minerals also act to shield ³⁵Cl from the thermal neutron flux, effectively suppressing the production of ³⁶Cl. To reconcile the measured ³⁶Cl ratios with the ratios calculated assuming chemical homogeneity, the shales were artificially split into three fractions: organic, clay mineral, and phosphate mineral. Neutron production was calculated separately for each fraction, and the calculation results demonstrated that the phosphate fraction exerted much more control on the ³⁶Cl ratio than the organic or clay mineral fractions. By varying the uranium and chlorine contents in the phosphate fraction, a new, heterogeneous ³⁶Cl ratio was calculated that agreed with the measured ratio for the overwhelming majority of the carbonaceous shales. When using rock chemistry to calculate the ³⁶Cl ratio, rock types that show mineralogical heterogeneity on a micron scale can be divided into bulk fractions and accessory fractions for separate calculations of neutron production and neutron absorption. In this manner, a more accurate, heterogeneous ³⁶Cl ratio can be calculated for the rock as a whole.     

Multicomponent reactive transport modeling of uranium bioremediation field experiments

A reaction network integrating abiotic and microbially mediated reactions has been developed to simulate biostimulation field experiments at a former Uranium Mill Tailings Remedial Action (UMTRA) site in Rifle, Colorado. The reaction network was calibrated using data from the 2002 field experiment, after which it was applied without additional calibration to field experiments performed in 2003 and 2007. The robustness of the model specification is significant in that (1) the 2003 biostimulation field experiment was performed with 3 times higher acetate concentrations than the previous biostimulation in the same field plot (i.e., the 2002 experiment), and (2) the 2007 field experiment was performed in a new unperturbed plot on the same site. The biogeochemical reactive transport simulations accounted for four terminal electron-accepting processes (TEAPs), two distinct functional microbial populations, two pools of bioavailable Fe(III) minerals (iron oxides and phyllosilicate iron), uranium aqueous and surface complexation, mineral precipitation and dissolution. The conceptual model for bioavailable iron reflects recent laboratory studies with sediments from the UMTRA site that demonstrated that the bulk (∼90%) of initial Fe(III) bioreduction is associated with phyllosilicate rather than oxide forms of iron. The uranium reaction network includes a U(VI) surface complexation model based on laboratory studies with Rifle site sediments and aqueous complexation reactions that include ternary complexes (e.g., calcium-uranyl-carbonate). The bioreduced U(IV), Fe(II), and sulfide components produced during the experiments are strongly associated with the solid phases and may play an important role in long-term uranium immobilization.     

Cation exchange capacity of loess and overlying soil in the non-carbonate loess sections, North-Western Croatia

The adsorption properties in terms of cation exchange capacity and their relation to the soil and sediment constituents (clay minerals, Fe-, Mn-, and Al-oxyhydroxides, organic matter) were investigated in loess, soil-loess transition zone, and soil at four loess-soil sections in North-Western Croatia. Cation exchange capacity of the bulk samples, the samples after oxalate extraction of Fe, Mn and Al, and after removal of organic matter, as well as of the separated clay fraction, was determined using copper ethylenediamine. Cation exchange capacity (pH～7) of the bulk samples ranges from 5 to 12 cmol _c /kg in soil, from 7 to 15 cmol _c /kg in the soil-loess transition zone, and from 12 to 20 cmol _c /kg in loess. Generally, CEC values increase with depth. Oxalate extraction of Fe, Mn, and Al, and removal of organic matter cause a CEC decrease of 3–38% and 8–55%, respectively, proving a considerable influence of these constituents to the bulk CEC values. In the separated clay fraction (<2 μm) CEC values are up to several times higher relative to those in the bulk samples. The measured CEC values of the bulk samples generally correspond to the clay mineral content identified. Also, a slight increase in muscovite/illite content with depth and the vermiculite occurrence in the loess horizon are concomitant with the CEC increase in deeper horizons, irrespective of the sample pretreatment.